1. Field of the Invention
The present invention relates to wireless communication system, and more particularly, to a method and apparatus for selecting a mobility management entity (MME) in a wireless communication system including a mobile relay node.
2. Related Art
Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
FIG. 1 shows network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice over internet protocol (VoIP) through IMS and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN), an evolved packet core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNB) 20, and a plurality of user equipment (UE) 10. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from eNB 20 to UE 10, and “uplink” refers to communication from the UE to an eNB. UE 10 refers to communication equipment carried by a user and may be also referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
An eNB 20 provides end points of a user plane and a control plane to the UE 10. MME/SAE gateway 30 provides an end point of a session and mobility management function for UE 10. The eNB and MME/SAE gateway may be connected via an S1 interface.
The eNB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNBs 20.
The MME provides various functions including NAS signaling to eNBs 20, NAS signaling security, AS security control, Inter CN node signaling for mobility between 3GPP access networks, Idle mode UE reachability (including control and execution of paging retransmission), tracking area list management (for UE in idle and active mode), PDN GW and serving GW selection, MME selection for handovers with MME change, SGSN selection for handovers to 2G or 3G 3GPP access networks, roaming, authentication, bearer management functions including dedicated bearer establishment, support for PWS (which includes ETWS and CMAS) message transmission. The SAE gateway host provides assorted functions including per-user based packet filtering (by e.g. deep packet inspection), lawful interception, UE IP address allocation, transport level packet marking in the downlink, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAE gateway 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.
A plurality of nodes may be connected between eNB 20 and gateway 30 via the S1 interface. The eNBs 20 may be connected to each other via an X2 interface and neighboring eNBs may have a meshed network structure that has the X2 interface.
FIG. 2 shows architecture of a typical E-UTRAN and a typical EPC.
As illustrated, eNB 20 may perform functions of selection for gateway 30, routing toward the gateway during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning of eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE_IDLE state management, ciphering of the user plane, system architecture evolution (SAE) bearer control, and ciphering and integrity protection of non-access stratum (NAS) signaling.
FIG. 3 shows a user-plane protocol and a control-plane protocol stack for the E-UMTS.
FIG. 3(a) is block diagram depicting the user-plane protocol, and FIG. 3(b) is block diagram depicting the control-plane protocol. As illustrated, the protocol layers may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based upon the three lower layers of an open system interconnection (OSI) standard model that is well known in the art of communication systems.
The physical layer, the first layer (L1), provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. Between different physical layers, namely, between physical layers of a transmission side and a reception side, data is transferred via the physical channel.
The MAC layer of Layer 2 (L2) provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 (L2) supports the transmission of data with reliability. It should be noted that the RLC layer illustrated in FIGS. 3(a) and 3(b) is depicted because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself is not required. The PDCP layer of Layer 2 (L2) performs a header compression function that reduces unnecessary control information such that data being transmitted by employing internet protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane and controls logical channels, transport channels and the physical channels in relation to the configuration, reconfiguration, and release of the radio bearers (RBs). Here, the RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the UTRAN.
As illustrated in FIG. 3(a), the RLC and MAC layers (terminated in an eNB 20 on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARQ). The PDCP layer (terminated in eNB 20 on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.
As illustrated in FIG. 3(b), the RLC and MAC layers (terminated in an eNodeB 20 on the network side) perform the same functions for the control plane. As illustrated, the RRC layer (terminated in an eNB 20 on the network side) may perform functions such as broadcasting, paging, RRC connection management, radio bearer (RB) control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway 30 on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE 10.
The RRC state may be divided into two different states such as a RRC_IDLE and a RRC_CONNECTED. In RRC_IDLE state, the UE 10 may receive broadcasts of system information and paging information while the UE specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area and may perform PLMN selection and cell re-selection. Also, in RRC_IDLE state, no RRC context is stored in the eNB.
In RRC_CONNECTED state, the UE 10 has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the network (eNB) becomes possible. Also, the UE 10 can report channel quality information and feedback information to the eNB.
In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE 10 belongs. Therefore, the network can transmit and/or receive data to/from UE 10, the network can control mobility (handover and inter-RAT cell change order to GERAN with NACC) of the UE, and the network can perform cell measurements for a neighboring cell.
In RRC_IDLE state, the UE 10 specifies the paging DRX cycle. Specifically, the UE 10 monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle.
The paging occasion is a time interval during which a paging signal is transmitted. The UE 10 has its own paging occasion.
A paging message is transmitted over all cells belonging to the same tracking area. If the UE 10 moves from one tracking area to another tracking area, the UE will send a tracking area update message to the network to update its location.
FIG. 4 shows an example of structure of a physical channel.
The physical channel transfers signaling and data between layer L1 of a UE and eNB. As illustrated in FIG. 4, the physical channel transfers the signaling and data with a radio resource, which consists of one or more sub-carriers in frequency and one more symbols in time.
One sub-frame, which is 1 ms in length, consists of several symbols. The particular symbol(s) of the sub-frame, such as the first symbol of the sub-frame, can be used for downlink control channel (PDCCH). PDCCHs carry dynamic allocated resources, such as PRBs and MCS.
A transport channel transfers signaling and data between the L1 and MAC layers. A physical channel is mapped to a transport channel.
Downlink transport channel types include a broadcast channel (BCH), a downlink shared channel (DL-SCH), a paging channel (PCH) and a multicast channel (MCH). The BCH is used for transmitting system information. The DL-SCH supports HARQ, dynamic link adaptation by varying the modulation, coding and transmit power, and both dynamic and semi-static resource allocation. The DL-SCH also may enable broadcast in the entire cell and the use of beamforming. The PCH is used for paging a UE. The MCH is used for multicast or broadcast service transmission.
Uplink transport channel types include an uplink shared channel (UL-SCH) and random access channel(s) (RACH). The UL-SCH supports HARQ and dynamic link adaptation by varying the transmit power and potentially modulation and coding. The UL-SCH also may enable the use of beamforming. The RACH is normally used for initial access to a cell.
The MAC sublayer provides data transfer services on logical channels. A set of logical channel types is defined for different data transfer services offered by MAC. Each logical channel type is defined according to the type of information transferred.
Logical channels are generally classified into two groups. The two groups are control channels for the transfer of control plane information and traffic channels for the transfer of user plane information.
Control channels are used for transfer of control plane information only. The control channels provided by MAC include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH) and a dedicated control channel (DCCH). The BCCH is a downlink channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging information and is used when the network does not know the location cell of a UE. The CCCH is used by UEs having no RRC connection with the network. The MCCH is a point-to-multipoint downlink channel used for transmitting MBMS control information from the network to a UE. The DCCH is a point-to-point bi-directional channel used by UEs having an RRC connection that transmits dedicated control information between a UE and the network.
Traffic channels are used for the transfer of user plane information only. The traffic channels provided by MAC include a dedicated traffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCH is a point-to-point channel, dedicated to one UE for the transfer of user information and can exist in both uplink and downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE.
Uplink connections between logical channels and transport channels include a DCCH that can be mapped to UL-SCH, a DTCH that can be mapped to UL-SCH and a CCCH that can be mapped to UL-SCH. Downlink connections between logical channels and transport channels include a BCCH that can be mapped to BCH or DL-SCH, a PCCH that can be mapped to PCH, a DCCH that can be mapped to DL-SCH, and a DTCH that can be mapped to DL-SCH, a MCCH that can be mapped to MCH, and a MTCH that can be mapped to MCH.
Meanwhile, 3GPP LTE-A may supports relaying by having a relay node (RN) wirelessly connect to an eNB serving the RN. It may be referred to Paragraph 4.7 of “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (release 10)” to 3GPP (3rd generation partnership project) TS 36.300 V10.2.0 (2010-12). The eNB serving the RN may be referred as donor eNB (DeNB). The DeNB and the RN may be connected via a modified version of the E-UTRA radio interface. The modified vision may be referred as a Un interface.
The RN may support eNB functionality. It means that the RN terminates the radio protocols of the E-UTRA radio interface, and an S1 and X2 interfaces. In addition to the eNB functionality, the RN may also support a subset of UE functionality, e.g, a physical layer, layer-2, RRC, and NAS functionality, in order to wirelessly connect to the DeNB.
FIG. 5 shows a block diagram illustrating network structure of an LTE-A system introducing a relay system.
Referring to FIG. 5, the LTE-A network includes an E-UTRAN, an EPC and one or more user equipment (not described). The E-UTRAN may include one or more eNB 111, one or more donor eNB (DeNB) 110, one or more relay node (RN) 100 and a plurality of UE may be located in one cell. One or more E-UTRAN MME/S-GW 120 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from the eNB 111 to the UE, from the DeNB 110 to the UE or from the RN 100 to the UE, and “uplink” refers to communication from the UE to the eNB 111, from the UE to the DeNB 110 or from the UE to the RN 100. The UE refers to communication equipment carried by a user and may be also referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
The eNB 111 and the DeNB 110 provide end points of a user plane and a control plane to the UE. MME/S-GW 120 provides an end point of a session and mobility management function for UE. The eNB 111 and the MME/S-GW 120 may be connected via an S1 interface. The DeNB 110 and MME/SAE gateway 120 may be connected via an S1 interface. The eNBs 111 may be connected to each other via an X2 interface and neighboring eNBs may have a meshed network structure that has the X2 interface. The eNB 111 and the DeNB 110 may be connected to each other via an X2 interface.
The RN 100 may be wirelessly connected to the DeNB 110 via a modified version of the E-UTRA radio interface being called the Un interface. That is, the RN 100 may be served by the DeNB 110. The RN 100 may support the eNB functionality which means that it terminates the S1 and X2 interfaces. Functionality defined for the eNB 111 or the DeNB 110, e.g. radio network layer (RNL) and transport network layer (TNL), may also apply to RNs 100. In addition to the eNB functionality, the RN 100 may also support a subset of the UE functionality, e.g. physical layer, layer-2, RRC, and NAS functionality, in order to wirelessly connect to the DeNB.
The RN 100 may terminate the S1, X2 and Un interfaces. The DeNB 110 may provide S1 and X2 proxy functionality between the RN 100 and other network nodes (other eNBs, MMEs and S-GWs). The S1 and X2 proxy functionality may include passing UE-dedicated S1 and X2 signaling messages as well as GTP data packets between the S1 and X2 interfaces associated with the RN 100 and the S1 and X2 interfaces associated with other network nodes. Due to the proxy functionality, the DeNB 110 appears as an MME (for S1) and an eNB (for X2) to the RN.
The DeNB 110 may also embed and provides the S-GW/P-GW-like functions needed for the RN operation. This includes creating a session for the RN 100 and managing EPS bearers for the RN 100, as well as terminating the S11 interface towards the MME serving the RN 100.
The RN and the DeNB may also perform mapping of signaling and data packets onto EPS bearers that are setup for the RN. The mapping may be based on existing QoS mechanisms defined for the UE and the P-GW.
The relay node may be classified to a fixed relay node and a mobile relay node. The mobile relay node may be applied to the 3GPP LTE rel-11. One of the possible deployment scenarios of mobile relay node is high speed public transportation, e.g, a high speed railway. That is, the mobile relay node may be put on the top of a high speed train. Hence, it is easily expected that the provision of various good quality services towards the users on a high speed public transportation will be important. Meanwhile, the service requirements offered by the fixed relay node seem to be different from those offered by the mobile relay node. So, there might be a few of considerations that should be resolved in the mobile relay node. The solutions to resolve these considerations for mobile relay node may have impacts on radio a radio access network (RAN).
A S1-flex concept provides support for network redundancy and load sharing of traffic across network elements in a core network (CN), the MME and the S-GW, by creating pools of MMEs and S-GWs and allowing each eNB to be connected to multiple MMEs and S-GWs in a pool. A DeNB supporting the mobile relay node can be connected to multiple MMEs. If the mobile relay node attaches to the DeNB, the DeNB needs to be connected to the MME which supports the mobile relay node. Accordingly, the DeNB needs to know that the relay node trying to attach to the DeNB is the mobile relay node, and whether the MME supports the mobile relay node or not.
A handover procedure may be supported in 3GPP LTE-A. Currently, in RRC_CONNECTED state, the network controls the handover procedure per UE basis. That is, the network decides the movement of each UE toward a new cell. The network triggers the handover procedure based on the radio conditions and load. When the mobile relay node is deployed, it is expected that handover occurs much more frequently, and the excessive signaling overhead will be incurred from per UE based handover. For example, massive UEs served by the mobile relay node may perform the handover procedure at the same time toward the same target eNB (or DeNB) when a high speed train having the mobile relay node stops at the station in the high speed railway scenario. Accordingly, the handover success rate will be reduced due to the excessive signaling overhead and the fact that tracking area update (TAU) is provided in a short period of time. Furthermore, UE measurements in high speed environments are typically less accurate than low speed environments. The UEs on the mobile relay node attached to the high speed public transportation will suffer from the reduced handover success rate. Accordingly, a group mobility handover procedure can be employed. A handover success rate can be improved by per group based handover instead of per UE based handover. When the mobile relay node tries to handover to the target DeNB, the target DeNB needs to be connected to the MME which supports the mobile relay node. Accordingly, the DeNB needs to know that the relay node trying to handover to the target DeNB is the mobile relay node, and whether the MME supports the mobile relay node or not.
Therefore, optimization is needed to resolve the problems.